Alzheimer's disease (AD) is the most commonly reported dementing illness, producing a progressive loss of memory and other higher cognitive functions, eventually leading to death. AD may only be definitively diagnosed by pathological observation of both senile plaques and neurofibrillary tangles. McKhann G, Drachman D, Folstein M, Katzman R, Price D, and Stadlan E M, Clinical Diagnosis of Alzheimer's Disease: Report of the NINCDS-ADRDA Work Group under the Auspices of Department of Health and Human Services Task Force on Alzheimer's Disease, Neurology 34:939–944 (1984); Khachaturian Z S, Diagnosis of Alzheimer's disease, Arch Neurol 42:1097–1105 (1985). Current methods of diagnosing AD at the early stages rely primarily on directly assessing patients and interviewing family members, often supplemented by neuroimaging studies. However, at specialist centers, evaluating for AD (clinical neuropsychometric and neuroradiological workups) takes several hours of the patient's and clinician's time; a quick and easy diagnostic test which would dramatically reduce time is still needed. Additionally, an early biological marker of AD would be useful so that treatments a slow the progression of AD or delay the onset of AD could be administered in the early stages of the disease, thereby optimizing therapeutic benefit. Brookmeyer R, and Zeger S, Statistical issues in prevention and therapeutic trials of Alzheimer's disease, Alzheimer Dis Assoc Disord 10:27–30 (1996); Brookmeyer R, Gray S, and Kawas C, Projection of Alzheimer's disease in the United States and the public health impact of delaying disease onset, Am J Publi Health 88:1337–1342 (1998). Consequently, a biomarker of AD that would allow for a rapid, simple, inexpensive, and minimally invasive substitute for the standard diagnostic assessment is still needed.
Most of the currently available commercial tests for diagnosing Alzheimer's Disease (AD) are Cerebral Spinal Fluid (CSF)-based, like ADmark tau/Aβ1-42 and Nymox AD7C-NTP and, therefore, are somewhat invasive. Mayeux R, Evaluation and use of diagnostic tests in Alzheimer's disease, Neurobiol Aging 19:139–143 (1998). A test which combines CSF tau and Aβ1-42 measurements and utilizes cutoff values which best separate AD cases from others has sensitivity values ranging between 50 and 85% and specificity values between 86 and 96%. Motter R, Vigo-Pelfrey C, and Kholodenko D, Reduction of β-amyloid peptide42 in the cerebrospinal fluid of patients with Alzheimer's disease, Annals Neurology 38:643–648(1995); Shoji M, Matsubara E, Kanai M, Watanabe M, Nakamura T, Tomidokoro Y, Shizuka M, Wakabayashi K, Igeta Y, Ikeda Y, Mizushima K, Amari M, Ishiguro K, Kawarabayashi T, Harigaya Y, Okamoto K, and Hirai S, Combination assay of CSF tau, A beta 1–40 and A beta 1–42(43) as a biochemical marker of Alzheimer's disease, J Neurol Sci 158:134–140 (1998); Tapiola T, Pirttila T, Mehta P D, Alafuzofff I, Lehtovirta M, and Soininen H, Relationship between apoE genotype and CSF beta-amyloid (1–42) and tau in patients with probable and definite Alzheimer's disease, Neurobiol Aging 21:735–740 (2000); Hulstaert F, Blennow K, Ivanoiu A, Schoonderwaldt H C, Riemenschneider M, De Deyn P P, Bancher C, Cras P, Wiltfang J, Mehta P D, Iqbal K, Pottel H, Vanmechelen E, and Vanderstichele H, Improved discrimination of AD patients using beta-amyloid (1–42) and tau levels in CSF, Neurology 52:1555–1562 (1999). However, critics have previously criticized the combined tau/Aβ1-42 test for its high false negative rate (which is due to AD cases with low CSF tau levels), and lack of diagnostic value when both tau/Aβ1-42 values are either high or low (approximately ⅓ of subjects). Ghanbari K, and Ghanbari H A, A sandwich enzyme immunoassay for measuring AD7C-NTP as an Alzheimer's disease marker: AD7C test, J Clin Lab Anal 6:379:383 (1998).
Another test, the AD7C-NTP biomarker test (Nymox corporation), is based on a raised neural thread protein in AD brains and CSF (although a large clinical study for the efficacy of measurement in urine is underway). Ghanbari K, and Ghanbari H A, A sandwich enzyme immunoassay for measuring AD7C-NTP as an Alzheimer's disease marker: AD7C test. J Clin Lab Anal 6:379:383 (1998); Ghanbari H, Ghanbari K, and Beheshti I, Biochemical assay for AD7C-NTP in urine as an Alzheimer's disease marker, J Clin Lab Anal 12:285:288 (1998). As a CSF-based test, the sensitivity and specificity were high (89%) for AD compared to age-matched, non-demented controls. Both the ADmark tau/Aβ1-42 and the Nymox AD7C-NTP tests were developed in known cases of dementia versus non-demented controls. A predictive test for cognitive impairment would also be useful, as subjects with cognitive impairment frequently progress to AD. Petersen R C, Mild cognitive impairment: transition between aging and Alzheimer's disease, Neurology 15:93–101 (2000).
The apolipoprotein E gene (APOE) has consistently been found to be associated with both sporadic and familial AD. Saunders A M, Strittmatter W J, Schmechel D, St. George-Hyslop P H, Pericak-Vance M A, Joo S H, Rosi B L, Gusella J F, Crapper-MacLachlan D R, Alberts M J, Hulette C, Crain B, Goldgaber D, and Roses A D, Association of apolipoprotein E allele epsilon 4 with late-onset familial and sporadic Alzheimer's disease, Neurology 43:1467–1472 (1993); Poirier J, Davignon J, Bouthillier D, Kogan S, Bertrand P, and Gauthier S, Apolipoprotein E polymorphism and Alzheimer's disease, Lancet 342:697–699 (1993). However, using APOE genotyping to diagnose AD is limited due to the non-Mendelian relationship between APOE genotype and AD. In a large autopsy-based study, using APOE genotyping alone (presence or absence of an ε4 allele) produced a 65% sensitivity and a 68% specificity. Mayeux R, Saunders A M, Shea S, Mirra S, Evans D, Roses A D, Hyman B T, Crain B, Tang M X, and Phelps C H, Utility of the apolipoprotein E genotype in the diagnosis of Alzheimer's disease, Alzheimer's disease centers consortium on apolipoprotein E and Alzheimer's disease, N Engl J Med 338:506–511 (1998). In the same study, the sequential use of clinical diagnosis and APOE genotyping reduced the false positive rate of the clinical diagnosis, increased specificity from 55% to 84% and modestly increasing the positive predictive value from 90 to 94%. Mayeux R, Evaluation and use of diagnostic tests in Alzheimer's disease, Neurobiol Aging 19:139–143 (1998); Mayeux R, Saunders A M, Shea S, Mirra S, Evans D, Roses A D, Hyman B T, Crain B, Tang M X, and Phelps C H, Utility of the apolipoprotein E genotype in the diagnosis of Alzheimer's disease, Alzheimer's disease centers consortium on apolipoprotein E and Alzheimer's disease, N Engl J Med 338:506–511 (1998). Thus, while using APOE alone as a predictive test is not recommended, a testing strategy that combines APOE and another biomarker may well be beneficial. Roses A D. Apoplipoprotein E genotyping in the differential diagnosis, not prediction, of Alzheimer's disease, Ann Neurol 38:6–14 (1995).
Aging remains the greatest risk factor for AD, and changes in peripheral immune cell phenotype associated with aging have been documented in previous studies. For example, an increase in the ratio of memory to naïve CD4+T cells (indicated by an increase in the CD45RO/CD45RA ratio), which reflects an increase in the activation status of these cells, has been shown to correlate with aging. Flurkey K, Stadecker M, and Miller RA, Memory T lymphocyte hyporesponsiveness to non-cognate stimuli: A key factor in age-related immunodeficiency, Eur J Immunol 22:931–935 (1992); Utsuyama M, Hirokawa K, Kurashima C, Fukayama M, Inamatsu T, Suzuki K, Hashimoto W, and Sato K, Differential age-change in the numbers of CD4+CD45RA and CD4+CD29+T cell subsets in human peripheral blood, Mech Ageing Dev 63:57–68 (1992). Similar shifts in the numbers and activation status of circulating T cell subpopulations have been postulated to account for a dysfunctional immune response observed in the elderly and even more so in AD patients. Hu G R, Walls R S, Creasey H, McCusker E, and Broe G A, Peripheral blood lymphocyte subset distribution and function in patients with Alzheimer's disease and other dementias, Aust N Z J Med 25:212–217 (1995); Ikeda T, Yamamoto K, Takahashi K, and Yamada M, Immune system-associated antigens on the surface of peripheral blood lymphocytes in patients with Alzheimer's disease, Acta Psychiatr Scand 83:444–448 (1991).